[libc] Add a linux Thread class in __support/threads.

This change is essentially a mechanical change which moves the thread
creation and join implementations from src/threads/linux to
src/__support/threads/linux/thread.h. The idea being that, in future, a
pthread implementation can reuse the common thread implementations in
src/__support/threads.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D123287